1. Field of the Invention
The invention relates to a method for transmitting a low-frequency signal over a data transmission link using a digital high bit-rate signal and to a transmitting device, as well as to a receiving device.
2. Description of Related Art
Especially in optical access point-to-point optical transmission systems, control data are transported in an optical overhead channel. An advantageous possibility of transmitting the overhead channel over the respective transmission link is to use the wanted high bit-rate digital signal of the data carrying channel as a carrier for a low-frequency signal (also designated as “tone”) including the control data. The tone is generated by amplitude (average power) modulation of the optical carrier signal using a small modulation index of typically less than 10 percent. The overhead channel information, that is the control data, is encoded in the amplitude or the phase of the tone.
In order to modulate the wanted high bit-rate digital signal according to the low-frequency signal (or tone), it is known to modulate the power of the optical carrier by modulating the bias current of the laser diode of the optical transmitting unit. However, this requires additional control hardware, which is not available in standard transmitter or transceiver modules. Alternatively, the tone can be impressed on the optical wanted high bit-rate digital signal by external modulation of the optical power. This requires a separate and costly external modulator, for example an electro-absorption modulator or a mechanical variable optical modulator.
It is thus an object of the present invention to provide a method for transmitting a low-frequency signal over a data transmission link using a digital high bit-rate signal that can be realized without additional hardware using the hardware of conventional transmitting and receiving (or transceiver) devices. It is a further object of the invention to provide a transmitting device and a receiving device suitable to implement this method.